In recent years, flat panel displays such as liquid crystal displays or plasma displays have been broadly used as image display devices in view of saving a space and realizing a high definition. Among these, liquid crystal displays have attracted attention due to a higher power saving ability and a high definition, and have been devoted to developing such liquid crystal displays.
To exhibits a function of liquid crystal display panels, polarizing plates serving as an optical shutter have been used along with liquid crystals in liquid crystal display panels. The polarizing plates which include a polarizer are an essential component for liquid crystal display panels. In general, the polarizers are prepared by uniaxial stretching of a polyvinyl alcohol (PVA) resin in a water bath with a stretch ratio of 5 times to 6 times, for that, it tend to be easily torn along the stretching direction and are fragile. For this reason, the polarizers have protective films formed on its one or/and both surfaces, and comprise the polarizing plates. In this case, an adhesive used to bond the protective film to the polarizers needs to satisfy specific properties to be required for the polarizing plates as well.
Conventionally, a water-based adhesive such as PVA, and the like has been used as an adhesive for preparing polarizing plates. However, the water-based adhesive has the problems of an improper in improvement of productivities and also exhibits a poor adhesive strength to the protective film made of cycloolefin polymer, and the like. To solve these problems, recently, an ultraviolet curable adhesive are used instead of the water-based adhesive for preparing the polarizing plates. As the ultraviolet curable adhesive, a cationic-curable adhesive comprised of a cationic polymerizable compound having an epoxy group, etc.; a radical-curable adhesive comprised of a radical polymerizable compound having a (meth)acryloyl group, etc.; and a so-called hybrid adhesive comprised of both the cationic polymerizable compound and the radical polymerizable compound; are used.
As an ultraviolet curable adhesive, for example, patent document 1 is about a cationic-curable adhesive comprising primarily (A) 100 parts by weight of an epoxy resin having two or more epoxy groups in a molecule, of which at least one is an alicyclic epoxy group, (B) 5 parts by weight to 1000 parts by weight of an epoxy resin having two or more epoxy groups in a molecule and substantially not having an alicyclic epoxy group and (C) 0.5 parts by weight to 20 parts by weight of a cationic photopolymerizable initiator. However, the cationic-curable adhesive disclosed in Japanese Patent Laid-Open Publication No. 2008-257199 A generally has the problems of leading to a reduction in productivity of the polarizing plates due to a reduced initial curability even though a high adhesion.
Patent document 2, discloses a radical-curable adhesive comprising a compound having a (meth)acryloyl group such as N-hydroxyethylacrylamide and N-acryloylmorpholine, and the like as a curable component. However, the radical-curable adhesive disclosed in Japanese Patent Laid-Open Publication No. 2008-287207 A generally has the problems of leading to a reduction in adhesion even though a high reactivity.
In addition, patent document 3 discloses a hybrid adhesive comprising primarily (A) 100 parts by weight of an active energy ray-curable compound comprising (A1) 5 wt % to 100 wt % of an active energy ray- and cationic-curable compound having an epoxy group or an oxetanyl group and not having an active energy ray and radical polymerizable functional group, with an weight average molecule weight of less than 5,000 and (A2) 0 wt % to 95 wt % of an active energy ray- and radical-curable compound and (B) 0.0001 part by weight to 2 parts by weight of an acryl resin having an epoxy group or an oxetanyl group with an weight average molecule weight of 5,000 to 150,000. However, the hybrid adhesive disclosed in Japanese Patent Laid-Open Publication No. 2012-172026 A generally has the problems of leading to a reduction in reliability such as an occurrence of cracks of the polarizers in a thermal shock test and also an insufficient adhesion.